1. Field of the Invention
The present invention relates to a positive active material for a rechargeable lithium battery and a method of preparing the same, and more particularly, to a positive active material for a rechargeable lithium battery having improved capability, power, and cycle-life characteristics, and a method of preparing the same.
2. Description of the Related Art
Generally, rechargeable lithium batteries use a material from or into which lithium ions are deintercalated or intercalated for positive and negative active materials. For an electrolyte, an organic solvent or a polymer is used. A rechargeable lithium battery produces electric energy as a result of changes in the chemical potentials of the active materials during the intercalation and deintercalation reactions of the lithium ions.
Such a rechargeable lithium battery, having an average discharge potential of 3.7 V (i.e., a battery having a substantially 4 V average discharge potential) is considered to be an essential element in the digital generation since it is an indispensable energy source for portable digital devices such as a cellular telephone, a notebook computer, a camcorder and so on (i.e., the “3C” devices).
For the negative active material in a rechargeable lithium battery, metallic lithium was used in the early days of development. Recently, however, carbon materials such as amorphous carbon or crystalline carbon-based materials, which reversibly intercalate lithium ions, are extensively used instead of the metallic lithium due to problems of high reactivity toward electrolyte and dendrite formation of the metallic lithium. With the use of carbon-based active materials, the potential safety problems that are associated with the metallic lithium can be prevented while achieving a relatively high energy density as well as a much improved cycle life. In particular, boron may be added to carbonaceous materials to produce boron-coated graphite (BOC) in order to increase the capability of the carbonaceous materials.
For the positive active material in the rechargeable lithium battery, chalcogenide compounds into or from which lithium ions are intercalated or deintercalated are used. Typical examples include LiCoO2, LiMn2O4, LiNiO2, LiNi1-xCoxO2 (1<x<1), and LiMnO2. Manganese-based positive materials such as LiMn2O4 or LiMnO2 are the easiest to prepare, are less expensive than the other materials, and are environmentally friendly. However, manganese-based materials have a disadvantage of a relatively low capability. LiNiO2 is inexpensive and has high capacity, but is difficult to prepare in the desired structure. LiCoO2 is relatively expensive, but is widely used and is commercially available as it has good electrical conductivity and high cell voltage. Although most of the current commercially available rechargeable lithium batteries (approximately 95%) use LiCoO2 as the positive active material, it is rather expensive. Therefore, there are considerable demands to find a less expensive alternative material.
As one scheme to satisfy such a demand, in U.S. Pat. No. 5,292,601, LixMO2 (wherein M is an element selected from Co, Ni, or Mn, x is 0.5−1) is suggested as an alternative material for LiCoO2 as the positive active material. U.S. Pat. No. 5,075,291 discloses a method of fabricating a rechargeable lithiated intercalation compound for the positive active material including mixing a coating composition comprising boron oxide, boric acid, lithium hydroxide, aluminum oxide, lithium aluminate, lithium metaborate, silicon dioxide, lithium silicate, or mixtures thereof with a lithiated intercalation compound in a particulate form, and fusing the coating compound at a temperature higher than about 400° C., thereby coating the particulate with the fused coating compound.
However, there are still further demands and desires for an improved positive active material having enhanced capacity, power, and cycle-life performance for batteries.